Electron discharge device including hollow cathode element for combined emission of spectral radiation and resonance detection

ABSTRACT

This invention relates to a spectral radiation device which includes a hollow cathode and an anode enclosed within an envelope for emitting a beam of spectral radiation and also for generating ground state atoms which will fluoresce and yield resonant radiation in response to selected incoming radiations.

United States Patent Inventors John D. Johnson Pittsburgh. Pa.; George K. Yarnasaki, Horsenheads, N.Y. Appl. No. 770,378 Filed Oct. 24, 1968 Patented June 8, 1971 Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

Continuation-impart of application Ser. No. 696,761, Jan. 10, 1968.

ELECTRON DISCHARGE DEVICE INCLUDING HOLLOW CATIIODE ELEMENT FOR COMBINED EMISSION OF SPECTRAL RADIATION AND RESONANCE DETECTION 9 Claims, 8 Drawing Figs.

U.S. Cl 356/82, 313/205, 313/209, 315/337, 356/87 Int. Cl G01] 3/30, HOli 17/06 Field of Search 313/209,

l 13,ss3,s10

[56] References Cited UNITED STATES PATENTS 2,433,809 12/1947 Clapp 313/209 3,305,746 2/1967 Walsh et al.. 3 1 3/2 10X 3,405,304 10/1968 Gillies et al. 356/87X 3,406,308 10/ l 968 Yamasaki 3 l 3/209X 3,475,099 10/1969 Yasuda et al. 3 l 3/209X FOREIGN PATENTS 374,889 6/1932 Great Britain 313/209 Primary Examiner-Roy Lake Assistant Examiner-Palmer C. Demeo Attorneys-F. H. Henson and C. F. Renz ABSTRACT: This invention relates to a spectral radiation device which includes a hollow cathode and an anode enclosed within an envelope for emitting a beam of spectral radiation and also for generating ground state atoms which will fluoresce and yield resonant radiation in response to selected incoming radiations.

ELECTRON DISCHARGE DEVICE INCLUDING HOLLOW CATHODE ELEMENT FOR COMBINED EMISSION OF SPECTRAL RADIATION AND RESONANCE DETECTION CROSS-REFERENCES TO RELATED APPLICATIONS This is a continuation-in-part of US. application Ser. No. 696,761, filed .lan. I0, 1968 entitled Electron Discharge Device Including Hollow Cathode Element For Combined Emission Of Spectral Radiation And Resonance Detection by John D. Johnson and assigned to the same assignee.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to electron discharge devices of the hollow cathode type which are adapted for emitting spectral radiation having defined spectral lines dependent upon the material within the cathode and for resonant detection of incoming beams of radiation containing spectral lines characteristic of the material of the cathode.

2. Description of the Prior Art In the field of atomic absorption spectroscopy, a hollow cathode tube may be used for an emission source or resonant detection. This subject is more thoroughly discussed in the parent case of this application. Generally, the constituents of an unknown chemical sample may be detected and determined by introducing a dispersed solution of an unknown sample into a suitable flame or other media to disassociate the sample into its atomic constituents. Radiation having a known spectral characteristic is directed through the vapor of the sample. Certain spectral lines of the beam of the radiation will be absorbed by the sample material if the vapor of the sample contains materials having the characteristic radiation of the radiations directed onto the vapor. The radiations after passing through the vapor may be analyzed to discover which spectral lines have been absorbed and the degree to which these 'lines have been absorbed. This will determine the constituents and the concentration of these constituents within the unknown sample.

Radiations with specific spectral lines for application to this type of spectroscopy may be provided by an electron discharge device of the hollow cathode type. In such devices, the cathode element is usually shaped in the form of a hollow cylinder having one end thereof closed. An anode element is disposed within the envelope to establish an electron discharge between the anode and the interior of the hollow cathode. The cathode element is made of a suitable conductive material either containing or provided therein of the material whose spectral lines are to be measured. A potential is applied between the anode and cathode elements to cause a flow of electrons therebetween. The anode and cathode elements are disposed within the envelope which is sealed with a suitable pressure of an inert gas which can be positively ionized by an electron discharge. The application of potential between the cathode and the anode generates an electron discharge which in turn assists in generation of positive gas ions which are attracted to and bombard the inner surface of the hollow cathode to thereby sputter atomic particles of the cathode material. The sputtered atomic particles of the metallic material are bombarded by the electrons and positive gas ions. This bombardment results in the sputtered atoms being excited from a neutral ground state to a higher energy level. When the sputtered atoms return to their ground state from a higher energy level to a lower energy level, the energy originally absorbed from the bombarding particles is released in the form of a specific radiation of wavelengths characteristic of the material in the cathode.

Recently, a process of analyzing the radiations after being directed through the vaporized sample has incorporated a resonance detector. These resonant detectors which may be in the form of a hollow cathode or a low-pressure thermal emitter generate a large field of ground state atoms of the desired element. The radiation to be analyzed is directed into the field of ground state atoms. If the radiation is of a wavelength or wavelengths of the ground state energy of the free atoms in the resonance detector, these atoms will be excited to a higher energy level and on return to a lower energy or ground level will emit radiations which may be detected by photocells. This is atomic fluorescence at resonant frequency. The parent case of this application discusses in more detail the general subject of the combination of a spectral emission source and resonant detector within the same envelope and utilizing a single hollow cathode.

It is a general object of the present invention to provide a new and improved spectral radiation device and resonant detector within the same envelope containing auxiliary electrodes, filaments, shields, apertures and optical filters so as to improve the intensity of the emitted ground state transitions and to also improve the signal-to-noise ratio in the detection of the atomic fluorescence from the resonance detector. By these several means, self-reversal of the emitted radiation is reduced and ground state atoms for resonant detection are concentrated at equilibrium in a region most free of the negative and positive glow regions of the device.

SUMMARY OF THE INVENTION 0 These and other objects are accomplished in accordance with the teachings of the present invention by providing a new and improved hollow cathode device containing within a single envelope means for generating spectral radiation of a specific radiation of wavelengths and for providing a cloud of ground state atoms for resonance detection.

DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is a diagrammatic view of an atomic absorption system for performing measurements and includes a dual function hollow cathode device embodying the teachings of this invention;

FIG. 2 is a sectional view of a modified hollow cathode device which may be incorporated into the system of FIG. 1;

FIG. 3 is a sectional view of another modified hollow cathode tube which may be incorporated into the system of FIG. 1;

FIG. 4 is a sectional view of another modified hollow cathode tube which may be incorporated into the system of FIG. 1;

FIG. 5 is a sectional view of another modification of a hollow cathode discharge device which may be incorporated into the system of FIG. 1;

FIG. 6 illustrates another modification of the hollow cathode discharge device which may be incorporated into the system of FIG. 1;

FIG. 7 illustrates another possible modification of a hollow cathode discharge device which may be incorporated into the system of FIG. 1; and

FIG. 8 illustrates another modification of a hollow cathode discharge device which may be incorporated into the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and in particular to FIG. I, there is shown an illustrative embodiment of a system incorporating the hollow cathode discharge device in accordance with the teachings of the present invention. The hollow cathode discharge device 10 includes an evacuated envelope 12 of a suitable insulating material such as glass. The envelope 12 includes a tubular body portion 14 having a window 16 of a suitable material transmissive to the spectral emission radiation involved such as quartz and with a base portion or tipoff portion 18 closing off the opposite end of the tubular member Id. In addition, a tubular window portion 20 is provided with its longitudinal axis perpendicular to the axis of the tubular body portion 14. The tubular portion 20 is also provided with a viewing window 22 which may also be ofa suitable transmissive material such as quartz. The window 22 is transmissive to the atomic fluorescence. A hollow cathode 24 is provided within the envelope and may be of a suitable material whose spectral line is desired. It may be a single element or an alloy comprising several elements of interest. The cathode 24 has an opening 26 provided therein. The material of interest must be in the wall of the opening 26. It may be desirable to make the remaining portion of a different material. A lead-in member 28 is provided from the base 18 for supporting the cathode 24 within the envelope. The hollow portion 26 of the cathode 24 faces the window 16. The cathode 24 is also positioned within the region defined between the window tubulation 20 and the base 18.

An anode 30 is positioned at a point slightly above the hollow cathode 24 and is supported within the envelope by means of a lead 32 which is secured to the base 18. An insulating sleeve 34 surrounds the entire length of the lead-in 32 from the base 18 to the active anode portion 30. A spacer member 38 of a suitable insulating material such as mica may be provided within the envelope as illustrated to additionally support the cathode 24 and the anode 30 and the outer periphery of the spacer 38 engages the inner surface of the envelope 10. In addition, a second spacer member 40 may be positioned as illustrated to engage the inner surface of the envelope and provide support for the anode as well as providing an aperture 42 to provide additional shielding of the cathode 24 from the anode 30 and also to provide optical shielding. This member 40 may be of a suitable insulating material such as silica, alumina, or porcelain. it is desirable that the anode 30 be disposed at a point remote with the viewing window 22.

An auxiliary electrode 50 may be provided within the envelope l2 and is a ring member which may be cylindrical in shape and which is tilted with respect to the axis of the tubular body portion 14. The auxiliary electrode 50 is positioned within a region of the tubular body portion 14 and defined by a continuation of the tubular window portion into the region defined by the body portion 14. The auxiliary electrode 50 may be of a similar material as the characteristic material such as magnesium within the cathode 24 so as to generate ground state atoms of the same material. The auxiliary electrode S0 is also provided with a lead 54 which passes through the spacers 38 and 40 to the base 18 and is supported by these members. A suitable potential source 56 is connected between the lead 32 and the lead 28 of the anode and cathode electrode respectively and may be of a potential of about 500 volts. In addition, a resistance 58 is connected in series between the source 56 and the cathode 24 and a resistance 60 is connected between the source 56 and the auxiliary electrode 50. An optical shielding member 70 having an opening 72 may also be provided within the tubular body portion 14 and centered with respect to the tubular window portion 20.

In the operation of the device as shown in FIG. 1, the potential applied by the source 56 to the cathode 24, the anode 30 and the auxiliary electrode 50 establishes a flow of electrons through the inert gas within the envelope from the auxiliary electrode 50 to the anode as well as from the cathode 24 to the anode 30. The auxiliary electrode 50 normally is operated at a potential more negative than the cathode 24. Due to the flow of electrons between the cathode 24 and anode 30, the gas is partially ionized and the positive ions are drawn into the hollow portion 26 of the cathode 24. The positive ions bombard the interior surface of the hollow portion 26 to sputter cathode particles into an emission region 80. The sputtered cathode particles are excited by other positive gas ions and the flow of electrons to emit spectral radiation having the characteristic wavelength of the magnesium of which the cathode 24 is made. The emission from this region 80 will pass through the output window 16 and will be hereinafter referred to as the spectral emission.

The flow of electrons between the auxiliary electrode 50 and the anode 30 will also partially ionize the gas and positive ions are drawn into the auxiliary electrode 50. These positive ions may cause the sputtering of cathode particles from the auxiliary electrode 50. These sputtered cathode particles which are small in number are excited by other positive ions and flow of electrons to emit a small amount of spectral radiation. in addition, this combination of the auxiliary electrode and the anode 30 will generate a relatively large number of ground state neutral atoms within a region 82. This function provides the necessary number of ground state atoms for resonant detection of the device in response to incoming or returned spectral radiations. These returning spectral radiations will in turn excite the ground state atoms to a higher level and on the return of these excited atoms to ground state, a radiation will be emitted which may be detected and viewed through the viewing window 22.

Radiation emitted from the region is focused by a suitable optical assembly as indicated by dashed lines by a suitable lens 55 into a sample region 57. Disassociated atoms of the material to be analyzed such as magnesium are generated within the region 57. For example, the sample material may be placed in solution and then vaporized in the region 57. The vaporized solution may be heated as by premixing laminar flow or direct consumption burner to disassociate the atoms of the sample material to thereby provide a cloud of atoms at a ground state level. Other means of disassociating the atoms of the sample include high temperature furnaces, arc plasmas, and high frequency gas plasmas. The beam of radiation emitted by the device 10 contains a specific spectral wavelength which may be effectively absorbed by the vapor within the region 57. More specifically, if the energy states of the disassociated atoms within the region 57 correspond to the wavelength of the spectral radiation from the region 80 in the device 10, the atoms will be excited thereby absorbing energy from the radiation. As shown in FIG. 1, the radiation passes through the region 57 and is reflected by a concave spherical mirror 59 along the path denoted by a dash-dot line back through the region 57 into the region 82 of the device 10. A suitable mechanical interrupter or chopper is provided in this system and includes a rotating sector 61 which is driven by a suitable motor 63. The sector 61 has open portions 65 which allow the radiation to pass therethrough and to be reflected by the mirror 59. The other portions of the sector 61 intercept the radiations during the other portion of the rotation of the sector 61.

There exists spurious and extraneous sources of radiation which may include scattered flame emission from the burner, fluorescence from scattered particles within the device 10, and resonant emission from the hollow cathode 24 of spectral wavelengths which are not absorbed by the region 52. During a portion of the rotation of the sector 61, the beam of radiation is allowed to be reflected back through the region 57 and the total radiation effect is detected. During the blocked part of the cycle of the rotation of the sector 61, detection will be made of the spurious sources of light. The amplitude of the alternating signal thus generated is a measure of the specific atomic absorption effect occurring in the region 57.

The beam of spectral radiation is reflected back through the region 57 and is focused by the lens 55 into the fluorescing region 82 lying within the region defined between the anode 30 and the auxiliary electrode 50. The resonant radiation generated in response to reflected spectral radiation may be observed through the window 22 of the tube. A suitable radiation sensor 68 such as a photocell may be used to observe the region 82. More specifically, a lens assembly 64 is used to focus the radiation derived from the region 82 onto the sensor 68 through a filter 66. it is noted that the sensor 68 may be exposed to reflected radiation from the hollow cathode element 24 or that of the fluorescent radiation from the region 82 may contain lines which are not of immediate interest and may be removed by means of the filter 66. In the case of multielement cathodes where resonant radiation of more than one element is of interest, filter 66 may be varied by replacement or more than one detector window, such as 22 may be provided. in

order to eliminate ambient radiation, the sensor 68 may be disposed within a container 72 which presents a black, matted surface to absorb ambient radiations.

The combination of the auxiliary electrode 50 and the anode 30 produce additional ground state population within the viewing region 82. This in effect increases the strength of the signal derived from the device 10. In addition, optical shielding in the form of the spacer 40 and the shield 70 further reduce the noise and thereby enhance the signal-to-noise ratio. The primary function of the shield 40 is to shield the negative glow region from the viewing window. The shield member 70 also provides the same function of eliminating the glow that may be found adjacent to the anode 30. It is desired that the detector 68 be focused to obtain the best signal from the region 50.

In FIG. 2 there is illustrated a modified hollow cathode discharge device in which the cathode 24 is positioned slightly off axis with regard to the axis of the envelope body 14 and with the anode 30 substantially on the axis of the envelope 14. The auxiliary electrode 50 is disposed on the opposite side of the axis with respect to the cathode 24 and positioned adjacent the tubular section 20. In this manner the spectral radiation may be emitted out of the window 16 from the cathode 24 and the returning radiation may be directed onto the region 82 adjacent the auxiliary electrode 50 where the large population of ground state atoms would be found. Excessive self-reversal of the emitted radiation is eliminated and such a structure also provides a good shielding of the spectral source from the radiation detector located exterior of the win dow 22. The same external potential connections are provided as in FIG. 1.

ln FlG. 3 a modified auxiliary electrode arrangement is illustrated. The auxiliary electrode 90 is a thermally heated resonance ring which is of the same material as the characteristic emission desired from the spectral radiation cathode 24. The ground state population is primarily generated in the device of FIG. 3 due to the heating of the auxiliary element 90 by means of a battery 92. The remaining portion of the source consisting of the cathodes 24 and 30 are similar to that previously described.

In all of the embodiments shown above a special window 22 has been illustrated. It is of course obvious that the envelope could be constructed of a suitable material so that the window 22 is not required and the radiation may be viewed directly through the envelope wall.

In FIG. 4, another modification of FIG. 1 is illustrated in which the auxiliary electrode 50 is omitted and an insulating sleeve 94 is provided which extends from the spacer member 38 and spacer member 41 upwardly above the cathode 24 and has an aperture 96 located in the side opposite the viewing window 22 for providing observation of the ground state atoms positioned within the area or region above the cathode 24. Such a shielding structure 94 permits a larger signal-tonoise ratio by selection of a resonant region of minimal incandescent radiation.

ln FIG. 5, a modified anode structure is illustrated and the structure consists of the cathode 24 positioned centrally within the envelope. The spacer 38 surrounds and supports the cathode 24. The spacer member 40 is positioned above the cathode 24 an annular disc member 98 serves as the anode for the cathode and is positioned above the cathode 24 and the spacer 40, an aperture 99 of the anode 98 is aligned with the opening in the hollow cathode 24. An additional shielding spacer 100 may be provided above the anode 98 as illustrated in which the shield 100 is also an annular disc member having an opening 101 of substantially the same dimensions as the aperture 99 in the anode 98. The additional spacer 100 may be provided above the anode to further enhance the signal-tonoise ratio of the device.

In FlG. 6, the anode 30 and cathode 24 are substantially conventional as that described with respect to FIG. 1 but in addition a series of shielding members 40, 104 and 106 are provided above the cathode 24 and are disc members having centrally located apertures.

In FIG. 7, a modified configuration is shown in which a cathode is a tubular member being open at both ends with mica support members 122 positioning the cathode 120 and shielding the exterior of cathode 120 from discharges. In this case, an annular anode 126 is positioned below the cathode 120 which substantially confines the spectral emission from the device to the lower portion of the cathode 120 and provides excellent shielding of the spectral radiation from the viewing window 22. Again an optical shield 128 may be desired above the cathode 120. The region of ground state atoms would be located above the shield 128 and the returning radiation would be directed into this region and viewed through the window 22.

In FIG. 8, there isillustrated a more or less conventional hollow cathode tube with a cathode 24 and an anode 30. A single window 16 is shown. The viewing is accomplished by locating a mirror 130 exterior of the envelope which may be moved or located to detect the most sensitive area or largest population of ground state atoms that are excited by the returning radiation.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. A dual purpose spectral analysis device including an envelope in which there is disposed a cathode and anode means for establishing an electron flow therebetween,

said envelope filled with a gas capable of supporting said electron flow and being capable of ionization, said cathode means having a hollow portion for concentrating a sputtering action causing a beam of spectral radiation characteristic of the material in said cathode means to be emitted and providing a cloud of ground state particles exterior of said hollow portion and characteristic of the material in said cathode means,

said envelope having a first and second portion transmissive respectively to said beam of spectral radiation and to a resonant fluorescence radiation derived from said cloud of ground state particles in response to an incoming resonant radiation,

an auxiliary electrode disposed within said envelope to enhance the cloud of ground state particles.

2. The device in claim 1 in which said auxiliary electrode operates at a potential negative with respect to said anode.

3. The device in claim 1 in which said auxiliary electrode operates at a potential negative with respect to said cathode.

4. The device in claim 1 in which said auxiliary electrode is an annular member to permit transmission of said spectral emission or said resonance fluorescence radiation therethrough.

5. The device in claim 1 in which said auxiliary electrode is heated to produce ground state atoms due to thermal emission.

6. The device in claim I in which an optical shield member is provided for substantially eliminating undesired radiation through said second portion while allowing transmission of said resonant fluorescence.

7. The device of claim 6 in which said optical shield member is an apertured member positioned within said envelope in which said shield is positioned to substantially maximize transmission of resonant fluorescence to said second portion while substantially reducing said spectral radiation.

8. The device in claim 1 in which detector means is provided responsive to said resonant fluorescence radiation and means is provided for selecting the desired portion of said cloud of ground state region to maximize the signal-to-noise ratio.

9. The device of claim 1 in which said sputtering action and associated beam of spectral radiation is positioned with respect to said cloud of ground state particles to minimize selfreversal. 

1. A dual purpose, spectral analysis device including an envelope in which there is disposed a cathode and anode means for establishing an electron flow therebetween, said envelope filled with a gas capable of supporting said electron flow and being capable of ionization, said cathode means having a hollow portion for concentrating a sputtering action causing a beam of spectral radiation characteristic of the material in said cathode means to be emitted and providing a cloud of ground state particles exterior of said hollow portion and characteristic of the material in said cathode means, said envelope having a first and second portion transmissive respectively to said beam of spectral radiation and to a resonant fluorescence radiation derived from said cloud of ground state particles in responSe to an incoming resonant radiation, an auxiliary electrode disposed within said envelope to enhance the cloud of ground state particles.
 2. The device in claim 1 in which said auxiliary electrode operates at a potential negative with respect to said anode.
 3. The device in claim 1 in which said auxiliary electrode operates at a potential negative with respect to said cathode.
 4. The device in claim 1 in which said auxiliary electrode is an annular member to permit transmission of said spectral emission or said resonance fluorescence radiation therethrough.
 5. The device in claim 1 in which said auxiliary electrode is heated to produce ground state atoms due to thermal emission.
 6. The device in claim 1 in which an optical shield member is provided for substantially eliminating undesired radiation through said second portion while allowing transmission of said resonant fluorescence.
 7. The device of claim 6 in which said optical shield member is an apertured member positioned within said envelope in which said shield is positioned to substantially maximize transmission of resonant fluorescence to said second portion while substantially reducing said spectral radiation.
 8. The device in claim 1 in which detector means is provided responsive to said resonant fluorescence radiation and means is provided for selecting the desired portion of said cloud of ground state region to maximize the signal-to-noise ratio.
 9. The device of claim 1 in which said sputtering action and associated beam of spectral radiation is positioned with respect to said cloud of ground state particles to minimize self-reversal. 